In recent years, semiconductor devices using metal-semiconducting barriers, known as Schottky barriers, instead of p/n junctions, have been developed to convert incident light, such as solar radiation, into electrical energy. Such devices, commonly referred to as Schottky-barrier type cells, are distinguishable from p/n junction devices by simplicity of fabrication, higher current output and improved radiation resistance.
Typically, a Schottky barrier type cell includes a substrate made from a single crystal wafer of gallium arsenide (GaAs). The bottom surface of the wafer is metallized to form one terminal of the cell and a thin, semiconducting layer of gallium arsenide is epitaxially grown on the top surface of the wafer. A layer of an oxide is deposited on top of the epitaxial gallium arsenside layer and a very thin layer of a noble metal is deposited on the oxide layer. Very thin layers (less than 100 angstrom units) of noble metals are able to transmit light. A metal grid electrode is then formed on the layer of noble metal to serve as the other terminal of the cell. Often, an anti-reflection coating is applied to the exposed surface of the metal layer.
Schottky barrier devices constructed in this manner suffer from a major defect: none of the noble metals adhere well to an oxide layer covering the epitaxial gallium arsenide layer. Efforts to avoid this defect by making devices without an oxide layer between the gallium arsenide epitaxial layer and the noble metal have been unsatisfactory because the noble metals adhere well to the epitaxial layer only after annealing, a step that usually destroys the stoichiometry of the epitaxial layer. Consequently, it is extremely difficult to either package or electrically couple an unannealed device to an external circuit because the noble metal layer tends to lift away from the device whenever an electrical lead is bonded to the grid electrode, thus causing an undesirable open circuit between the metal layer and the device.